Ofgem itself has published conflicting statements about whether the SMI reflects profits. In a consumer factsheet published on Thursday entitled “understanding the profits of the large energy suppliers”, it explains that the SMI “give the latest information on energy prices and profit margins”.

But the regulator told The Sunday Telegraph that “the SMI does not show profits”.

The row over Ofgem’s data comes ahead of a parliamentary hearing on Tuesday when executives of the Big Six companies will appear before MPs on the Energy Select Committee to discuss “profits, prices and poverty”.

The companies are expected to face questions over whether the levels of profits are fair, whether they should be doing more to shield consumers from rising prices, and whether they pay enough tax in the UK.

In written evidence to the MPs, British Gas said: “Erroneous assumptions in Ofgem’s [SMI] methodology materially overstate reported margins by around £35 - £75.” It warned that “trust in the energy sector is being undermined as a result” and said the regulator must urgently review its system.

Centrica said British Gas’s post-tax profit margins — typically the highest in the industry — had averaged 5pc for the past five years and were currently the equivalent of £50 per customer.

SSE, which also raises the issue with MPs, criticised the SMI in a blog. Deputy chief executive Alistair Phillips-Davies wrote: “Ofgem’s methodology appears to have a consistent bias towards overstating profit margins.”

He pointed out that, over 2010 and 2011, “Ofgem’s [annual] estimated profit margins averaged £73 per customer”. But a “quick comparison” with Ofgem-approved accounts for the period shows that on average they actually earned £30 per customer.

The Ofgem factsheet published last week shows that the Big Six suppliers’ average pre-tax profit margins fell to 3.1pc in 2011. It also says that the SMI “suggest that margins and consumer bills have increased since 2011”.

Its latest SMI says: “The rolling average net margin for a typical, standard-tariff, dual-fuel customer is approximately £95.” That implies a net margin of 6.7pc, based on Ofgem’s figure of £1,420 for a typical bill.

An Ofgem spokesman said yesterday it was incorrect to compare historical data with the SMI. “Our indicator represents a snapshot estimate of the prices that suppliers charge their customers and the costs that suppliers face – this gives an indication of profitability in the market.

“Our analysis is for a typical customer on a standard variable tariff... It is not an attempt to model actual profits of individual companies.” Ofgem had “consulted widely on our methodology” for SMI, he added.

The Big Six suppliers are all expected to come out strongly against charges of profiteering at Tuesday’s hearing and have used written evidence to insist their profit margins are fair.

SSE has told the committee it wants to increase its margin to about 5pc in the medium term, which it said was “reasonable”, lower than sectors such as telecoms, and necessary to justify investments in power plants.

The Big Six are preparing for scrutiny over the tax they pay. Centrica and SSE have revealed high tax contributions, but critics say the accounts of the other four, foreign-owned suppliers — E.On, nPower, ScottishPower and EDF Energy — are less clear. It is thought one of the firms that has invested heavily in new power plants has used legitimate allowances to cut its tax liability.